migrations: Replace deprecated django.db.backends.postgresql_psycopg2.

This has been a deprecated alias for django.db.backends.postgresql
since Django 1.9, removed in Django 3.0.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/migrations/0277_migrate_alert_word.py

@@ -1,5 +1,5 @@
 from django.db import migrations
-from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 import ujson
 from typing import Dict, List